FIGURE 18-10 (part 2) Urea cycle and reactions that feed amino groups into the cycle. The enzymes catalyzing these reactions (named in the text) are distributed between the mitochondrial matrix and the cytosol. One amino group enters the urea cycle as carbamoyl phosphate, formed in the matrix; the other enters as aspartate, formed in the matrix by transamination of oxaloacetate and glutamate, catalyzed by aspartate aminotransferase. The urea cycle consists of four steps. 1 Formation of citrulline from ornithine and carbamoyl phosphate (entry of the first amino group); the citrulline passes into the cytosol. 2 Formation of argininosuccinate through a citrullyl-AMP intermediate (entry of the second amino group). 3 Formation of arginine from argininosuccinate; this reaction releases fumarate, which enters the citric acid cycle. 4 Formation of urea; this reaction also regenerates ornithine. The pathways by which NH4+ arrives in the mitochondrial matrix of hepatocytes were discussed in Section 18.1. 2

Entry of Aspartate into the Urea Cycle This is the second nitrogen-acquiring reaction

MECHANISM FIGURE 18-11b Nitrogen-acquiring reactions in the synthesis of urea. The urea nitrogens are acquired in two reactions, each requiring ATP. (b) In the reaction catalyzed by argininosuccinate synthetase, the second nitrogen enters from aspartate. Activation of the ureido oxygen of citrulline in step 1 sets up the addition of aspartate in step 2. Nitrogen-acquiring reactions in the synthesis of urea. In the reaction catalyzed by argininosuccinate synthetase, the second nitrogen enters from aspartate. Activation of the ureido oxygen of citrulline in step 1 sets up the addition of aspartate in step 2. 4

Aspartate –Arginosuccinate Shunt Links Urea Cycle and Citric Acid Cycle FIGURE 18-12 Links between the urea cycle and citric acid cycle. The interconnected cycles have been called the "Krebs bicycle." The pathways linking the citric acid and urea cycles are known as the aspartate-argininosuccinate shunt; these effectively link the fates of the amino groups and the carbon skeletons of amino acids. The interconnections are even more elaborate than the arrows suggest. For example, some citric acid cycle enzymes, such as fumarase and malate dehydrogenase, have both cytosolic and mitochondrial isozymes. Fumarate produced in the cytosol—whether by the urea cycle, purine biosynthesis, or other processes—can be converted to cytosolic malate, which is used in the cytosol or transported into mitochondria (via the malateaspartate shuttle; see Figure 19-29) to enter the citric acid cycle. 5

Not All Amino Acids can be Synthesized in Humans These amino acids must be obtained as dietary protein Consumption of a variety of foods (including vegetarian only diets) well supplies all the essential amino acids

TABLE 18-1 Nonessential and Essential Amino Acids for Humans and the Albino Rat 7

Fate of Individual Amino Acids Seven to acetyl-CoA Leu, Ile, Thr, Lys, Phe, Tyr, Trp Six to pyruvate Ala, Cys, Gly, Ser, Thr, Trp Five to -ketoglutarate Arg, Glu, Gln, His, Pro Four to succinyl-CoA Ile, Met, Thr, Val Two to fumarate Phe, Tyr Two to oxaloacetate Asp, Asn

Summary of Amino Acid Catabolism FIGURE 18-15 Summary of amino acid catabolism. Amino acids are grouped according to their major degradative end product. Some amino acids are listed more than once because different parts of their carbon skeletons are degraded to different end products. The figure shows the most important catabolic pathways in vertebrates, but there are minor variations among vertebrate species. Threonine, for instance, is degraded via at least two different pathways (see Figure 18-19, 18-27), and the importance of a given pathway can vary with the organism and its metabolic conditions. The glucogenic and ketogenic amino acids are also delineated in the figure, by color shading. Notice that five of the amino acids are both glucogenic and ketogenic. The amino acids degraded to pyruvate are also potentially ketogenic. Only two amino acids, leucine and lysine, are exclusively ketogenic. Summary of Amino Acid Catabolism 9

Some enzyme cofactors important in one-carbon transfer reactions FIGURE 18-16 Some enzyme cofactors important in one-carbon transfer reactions. The nitrogen atoms to which one-carbon groups are attached in tetrahydrofolate are shown in blue. Some enzyme cofactors important in one-carbon transfer reactions

FIGURE 18-16 (part 2) Some enzyme cofactors important in one-carbon transfer reactions. The nitrogen atoms to which one-carbon groups are attached in tetrahydrofolate are shown in blue.

Conversions of one-carbon units on tetrahydrofolate FIGURE 18-17 Conversions of one-carbon units on tetrahydrofolate. The different molecular species are grouped according to oxidation state, with the most reduced at the top and most oxidized at the bottom. All species within a single shaded box are at the same oxidation state. The conversion of N5,N10-methylenetetrahydrofolate to N5-methyltetrahydrofolate is effectively irreversible. The enzymatic transfer of formyl groups, as in purine synthesis (see Figure 22-33) and in the formation of formylmethionine in bacteria (Chapter 27), generally uses N10-formyltetrahydrofolate rather than N5-formyltetrahydrofolate. The latter species is significantly more stable and therefore a weaker donor of formyl groups. N5-Formyltetrahydrofolate is a minor byproduct of the cyclohydrolase reaction, and can also form spontaneously. Conversion of N5-formyltetrahydrofolate to N5,N10-methenyltetrahydrofolate requires ATP, because of an otherwise unfavorable equilibrium. Note that N5-formiminotetrahydrofolate is derived from histidine in a pathway shown in Figure 18-26.

Synthesis of methionine and S-adenosylmethionine in an activated-methyl cycle FIGURE 18-18 Synthesis of methionine and S-adenosylmethionine in an activated-methyl cycle. The steps are described in the text. In the methionine synthase reaction (step 4), the methyl group is transferred to cobalamin to form methylcobalamin, which in turn is the methyl donor in the formation of methionine. S-Adenosylmethionine, which has a positively charged sulfur (and is thus a sulfonium ion), is a powerful methylating agent in several biosynthetic reactions. The methyl group acceptor (step 2) is designated R.

Catabolic pathways for alanine, glycine, serine, cysteine, tryptophan, and threonine FIGURE 18-19 Catabolic pathways for alanine, glycine, serine, cysteine, tryptophan, and threonine. The fate of the indole group of tryptophan is shown in Figure 18-21. Details of most of the reactions involving serine and glycine are shown in Figure 18-20. The pathway for threonine degradation shown here accounts for only about a third of threonine catabolism (for the alternative pathway, see Figure 18-27). Several pathways for cysteine degradation lead to pyruvate. The sulfur of cysteine has several alternative fates, one of which is shown in Figure 22-15. Carbon atoms here and in subsequent figures are color-coded as necessary to trace their fates.

FIGURE 18-19 (part 1) Catabolic pathways for alanine, glycine, serine, cysteine, tryptophan, and threonine. The fate of the indole group of tryptophan is shown in Figure 18-21. Details of most of the reactions involving serine and glycine are shown in Figure 18-20. The pathway for threonine degradation shown here accounts for only about a third of threonine catabolism (for the alternative pathway, see Figure 18-27). Several pathways for cysteine degradation lead to pyruvate. The sulfur of cysteine has several alternative fates, one of which is shown in Figure 22-15. Carbon atoms here and in subsequent figures are color-coded as necessary to trace their fates.

FIGURE 18-19 (part 2) Catabolic pathways for alanine, glycine, serine, cysteine, tryptophan, and threonine. The fate of the indole group of tryptophan is shown in Figure 18-21. Details of most of the reactions involving serine and glycine are shown in Figure 18-20. The pathway for threonine degradation shown here accounts for only about a third of threonine catabolism (for the alternative pathway, see Figure 18-27). Several pathways for cysteine degradation lead to pyruvate. The sulfur of cysteine has several alternative fates, one of which is shown in Figure 22-15. Carbon atoms here and in subsequent figures are color-coded as necessary to trace their fates.

MECHANISM FIGURE 18-20 Interplay of the pyridoxal phosphate and tetrahydrofolate cofactors in serine and glycine metabolism. The first step in each of these reactions (not shown) involves the formation of a covalent imine linkage between enzyme-bound PLP and the substrate amino acid—serine in (a), glycine in (b) and (c). (a) A PLP-catalyzed elimination of water in the serine dehydratase reaction (step 1) begins the pathway to pyruvate. (b) In the serine hydroxymethyltransferase reaction, a PLP-stabilized carbanion (product of step 1) is a key intermediate in the reversible transfer of the methylene group (as —CH2—OH) from N5,N10-methylenetetrahydrofolate to form serine. (c) The glycine cleavage enzyme is a multienzyme complex, with components P, H, T, and L. The overall reaction, which is reversible, converts glycine to CO2 and NH4+, with the second glycine carbon taken up by tetrahydrofolate to form N5,N10-methylenetetrahydrofolate. Pyridoxal phosphate activates the α carbon of amino acids at critical stages in all these reactions, and tetrahydrofolate carries one-carbon units in two of them (see Figure 18-6, 18-17). Interplay of the pyridoxal phosphate and tetrahydrofolate cofactors in serine and glycine metabolism

TABLE 18-2 Some Human Genetic Disorders Affecting Amino Acid Catabolism

FIGURE 18-21 Catabolic pathways for tryptophan, lysine, phenylalanine, tyrosine, leucine, and isoleucine. These amino acids donate some of their carbons (red) to acetyl-CoA. Tryptophan, phenylalanine, tyrosine, and isoleucine also contribute carbons (blue) to pyruvate or citric acid cycle intermediates. The phenylalanine pathway is described in more detail in Figure 18-23. The fate of nitrogen atoms is not traced in this scheme; in most cases they are transferred to α-ketoglutarate to form glutamate. Catabolic pathways for tryptophan, lysine, phenylalanine, tyrosine, leucine, and isoleucine.

FIGURE 18-21 (part 1) Catabolic pathways for tryptophan, lysine, phenylalanine, tyrosine, leucine, and isoleucine. These amino acids donate some of their carbons (red) to acetyl-CoA. Tryptophan, phenylalanine, tyrosine, and isoleucine also contribute carbons (blue) to pyruvate or citric acid cycle intermediates. The phenylalanine pathway is described in more detail in Figure 18-23. The fate of nitrogen atoms is not traced in this scheme; in most cases they are transferred to α-ketoglutarate to form glutamate.

FIGURE 18-21 (part 2) Catabolic pathways for tryptophan, lysine, phenylalanine, tyrosine, leucine, and isoleucine. These amino acids donate some of their carbons (red) to acetyl-CoA. Tryptophan, phenylalanine, tyrosine, and isoleucine also contribute carbons (blue) to pyruvate or citric acid cycle intermediates. The phenylalanine pathway is described in more detail in Figure 18-23. The fate of nitrogen atoms is not traced in this scheme; in most cases they are transferred to α-ketoglutarate to form glutamate.

Tryptophan as precursor FIGURE 18-22 Tryptophan as precursor. The aromatic rings of tryptophan give rise to nicotinate (niacin), indoleacetate, and serotonin. Colored atoms trace the source of the ring atoms in nicotinate. Tryptophan as precursor

FIGURE 18-23 Catabolic pathways for phenylalanine and tyrosine FIGURE 18-23 Catabolic pathways for phenylalanine and tyrosine. In humans these amino acids are normally converted to acetoacetyl-CoA and fumarate. Genetic defects in many of these enzymes cause inheritable human diseases (shaded yellow). Catabolic pathways for phenylalanine and tyrosine. In humans these amino acids are normally converted to acetoacetyl-CoA and fumarate. Genetic defects in many of these enzymes cause inheritable human diseases.

FIGURE 18-24 Role of tetrahydrobiopterin in the phenylalanine hydroxylase reaction. The H atom shaded pink is transferred directly from C-4 to C-3 in the reaction. This feature, discovered at the National Institutes of Health, is called the NIH shift. Role of tetrahydrobiopterin in the phenylalanine hydroxylase reaction. The H atom shaded pink is transferred directly from C-4 to C-3 in the reaction. This feature, discovered at the National Institutes of Health, is called the NIH shift.

In PKU, phenylpyruvate accumulates in the tissues, blood, and urine In PKU, phenylpyruvate accumulates in the tissues, blood, and urine. The urine may also contain phenylacetate and phenyllactate. FIGURE 18-25 Alternative pathways for catabolism of phenylalanine in phenylketonuria. In PKU, phenylpyruvate accumulates in the tissues, blood, and urine. The urine may also contain phenylacetate and phenyllactate.

Catabolic pathways for arginine, histidine, glutamate, glutamine, and proline FIGURE 18-26 Catabolic pathways for arginine, histidine, glutamate, glutamine, and proline. These amino acids are converted to α-ketoglutarate. The numbered steps in the histidine pathway are catalyzed by 1 histidine ammonia lyase, 2 urocanate hydratase, 3 imidazolonepropionase, and 4 glutamate formimino transferase.

Catabolic pathways for methionine, isoleucine, threonine, and valine. FIGURE 18-27 Catabolic pathways for methionine, isoleucine, threonine, and valine. These amino acids are converted to succinyl-CoA; isoleucine also contributes two of its carbon atoms to acetyl-CoA (see Figure 18-21). The pathway of threonine degradation shown here occurs in humans; a pathway found in other organisms is shown in Figure 18-19. The route from methionine to homocysteine is described in more detail in Figure 18-18; the conversion of homocysteine to α-ketobutyrate in Figure 22-14; the conversion of propionyl-CoA to succinyl-CoA in Figure 17-11.

Catabolic pathways for the three branchedchain amino acids: valine, isoleucine, and leucine. All three pathways occur in extrahepatic tissues and share the first two enzymes, as shown here. The branched-chain α-keto acid dehydrogenase complex is analogous to the pyruvate and α-ketoglutarate dehydrogenase complexes.This enzyme is defective in people with maple syrup urine disease. FIGURE 18-28 Catabolic pathways for the three branchedchain amino acids: valine, isoleucine, and leucine. All three pathways occur in extrahepatic tissues and share the first two enzymes, as shown here. The branched-chain α-keto acid dehydrogenase complex is analogous to the pyruvate and α-ketoglutarate dehydrogenase complexes and requires the same five cofactors (some not shown here). This enzyme is defective in people with maple syrup urine disease.

Catabolic pathway for asparagine and aspartate FIGURE 18-29 Catabolic pathway for asparagine and aspartate. Both amino acids are converted to oxaloacetate. Catabolic pathway for asparagine and aspartate

Chapter 18: Summary In this chapter, we learned that: Amino acids from protein are an important energy source in carnivorous animals Catabolism of amino acids involves transfer of the amino group via PLP-dependent aminotransferase to a donor such as -ketoglutarate to yield L-glutamine L-glutamine can be used to synthesize new amino acids, or it can dispose of excess nitrogen as ammonia In most mammals, toxic ammonia is quickly recaptured into carbamoyl phosphate and passed into the urea cycle